The present invention relates to gas turbine engines and more particularly, to the compressor casing designs of aircraft gas turbine engines.
A typical aircraft gas turbine engine includes a compressor for pressurizing air which subsequently is mixed with fuel and ignited in the combustor section and the resulting combustion gases power the turbine. The compressor and turbine are surrounded by casings through which air is either extracted or distributed. Typically, a portion of the air is extracted from the compressor as bleed air and is used to cool engine parts such as the turbines, blades, bearings, and gears, as well as to supply cabin pressure to the aircraft.
In a typical compressor, an inner casing or flow path is concentric with an outer casing and mounts stator vanes. The inner casing also surrounds the rotor blades of the compressor. Bleed air extracted from the compressor is typically channeled through the inner casing, gathered into a plenum formed between the inner and outer casings, and exited through pipes in the outer compressor casing.
One prior art device for extracting or bleeding air includes providing the inner casing with bleed holes or slots which channel a portion of the compressor air into the plenum. The bleed air then flows circumferentially in the plenum to the nearest exit pipe in the outer casing where it is discharged from the compressor. A problem with this method of extraction is that, as the air flows from the supply slots in the inner casing to the exit pipes in the outer casing, the air velocity increases. Accordingly, the heat transfer capability of the bleed air, which is directly proportional to its velocity, is relatively low at the midpoint between two exit pipes and relatively high at the exit pipes.
As the bleed air heats the outer casing, the portion of the outer casing adjacent the exit pipes will expand more rapidly than the portion of the casing between the exit pipes, and this uneven heating causes the outer casing to distort and become out-of-round. Since the inner casing is supported by the outer casing, the inner casing also becomes out-of-round. This distortion is undesirable since the inner casing surrounds the rotor blades, and increased clearance between the blade tips and the inner casing results in a decrease of compressor efficiency.
Applicant has previously designed an air channeling assembly for bleeding air from a compressor for reducing circumferential thermal distortion of the casings. The air channeling assembly includes a plurality of baffles disposed in the plenum and spaced radially inwardly from the outer casing. Each of the baffles includes an intermediate opening communicating with a different one of the exit pipes, and opposing ends extending circumferentially away from the intermediate opening. The baffles provide a flow shield between the inner and outer casings around each of the exit pipes and channel air through the intermediate opening to the associated exit pipe.
Although this assembly reduces circumferential thermal distortion of the compressor casing, the assembly adds complexity and expense to the manufacture of the compressor, as well as added weight to the compressor. Therefore, a need exists for an apparatus for reducing circumferential thermal distortion of the compressor casings that does not add complexity, expense or additional weight to the compressor.